With the migration from monochrome displays toward higher resolution color displays, a white backlight to properly display colors has been required. There are two main methods for providing a white light source: white LEDs and cold-cathode fluorescent lamps (CCFLs). CCFLs have been used for years in notebook computers. However, because of their size, complexity, and cost advantages, white LEDs are becoming the preferred light source for small handheld devices.
White LEDs can be powered in parallel or in series. A weakness of the parallel method is that white LED current and brightness do not necessarily match. For the parallel connected white LEDs, forward voltage variance of these white LEDs can be greater than 100 mV. This can lead to poor current and brightness matching in parallel configurations where currents across the white LEDs are not regulated. Generally, a current source is used to regulate the current through a white LED. Another problem is that the white LEDs have a high voltage drop, 3.1V to 3.4V depending on manufacturers, compared to red and green LEDs with a typical voltage drop of 1.8V to 2.7V. Whereas the red and green LEDs can be powered directly from a common battery, the white LEDs require the battery voltage to be regulated. Usually, a DC-DC converter is used to regulate the driving voltage of the white LEDs.
With regard to the efficiency, power loss in white LED drivers typically occurs in power consumed by DC-DC converters and power consumed by white LED current sources. Typically, the power loss from the white LED current source dictates the white LED driver efficiency. It is the goal of both applications and integrated circuit (IC) design to find ways to minimize the power loss in the white LED current source by reducing the voltage drop across the current source. In conventional designs, the voltage drop is kept constant for all possible load conditions, which will lead to a considerable variation in efficiency under different load conditions. Furthermore, each white LED is usually provided with an exclusive current source. When the number of white LEDs is large, the current sources will occupy a considerable area. This will undermine the pursuit for a small die area during the design.
Accordingly, it is to such an apparatus and method that can provide a high efficiency and at the same time reduce the die area the present invention is primarily directed.